In general terms, a transformer is a power converter that transfers alternating current (AC) electrical energy through inductive coupling between circuits of the transformer's windings.
Dry-type transformers are typically used for voltages up to 36 kV. They are mostly equipped with off-load tap changers allowing to set five different voltage ratios and a range of +/−5%. On-load tap changers are rarely used with dry-type transformers. Currently the application range of dry type transformer designs is being extended, involving a significant increase of their voltage rating. At this voltage levels most applications require the use of an on-load tap changer (OLTC) with much larger regulation range (+/−20%) and number of steps, as well as a corresponding extended regulation winding.
In the oil and gas industry, electric motors are used to drive submersible pumps which are located down in an oil or gas well. Such a motor is typically energized through a transformer connected at the well site to a conventional power distribution network.
Dry-type transformers have been operated at low voltage levels and with a small regulation range; in this case the voltages related to the transient oscillations can easily be managed and require relatively small dielectric distances. However, with increasing voltage and regulation range, the insulation distances grow and larger and larger dimensions are required also for the OLTC. Particularly, during impulse tests, transient oscillations are excited in the regulating winding of dry type transformers, which lead to high electric stresses on the OLTC. These stresses are particularly pronounced for a simple linear tap changer concept and when the OLTC is in the minimum position, so that the whole regulating winding is open (i.e., connected to the main winding at one end only).
U.S. Pat. No. 5,005,100 A discloses a transformer that comprises a primary winding and a secondary winding and that also includes a capacitor connected across at least a portion of the secondary winding within a housing of the transformer so that magnetically coupled voltage transients are filtered to prevent such transients from damaging a load connected to the secondary winding. An electrostatic shield is also included in the transformer to shield against capacitively coupled voltage transients. The capacitor is said also to improve the power factor.
EP 0 078 985 A1 relates to internal voltage grading and transient voltage protection for power transformer windings. A series string of plural zinc oxide varistor elements is electrically connected across each winding of a power transformer, with interior winding taps being electrically connected to the junctions between varistor elements. Each varistor string, disposed within the transformer casing, protects its associated winding from voltage surges in the same manner as externally mounted lightning arresters, provides highly effective voltage grading, and suppresses harmful transient voltage oscillations between the winding taps.
EP 0 187 983 A1 relates to a filter circuit including ZnO overvoltage arresters. A filter circuit with zinc oxide surge diverters for protection against transient interference and surges is connected in an alternating voltage network.
Hence, there is still a need for an improved transformer arrangement for mitigating transient voltage oscillations.